ui: defer setting pager related properties until the pager has spawned

ui: defer setting pager related properties until the pager has spawned

When --pager=on is given, dispatch.py spawns a pager before setting up color.
If the pager failed to launch, ui.pageractive was left set to True, so color
configured itself based on 'color.pagermode'.  A typical MSYS setting would be
'color.mode=auto, color.pagermode=ansi'.  In the failure case, this would print
a warning, disable the pager, and then print the raw ANSI codes to the terminal.

Care needs to be taken, because it appears that leaving ui.pageractive=True was
the only thing that prevented an attempt at running the pager again from inside
the command.  This results in a double warning message, so pager is simply
disabled on failure.

The ui config settings didn't need to be moved to fix this, but it seemed like
the right thing to do for consistency.

--- a/mercurial/chgserver.py	Sat Mar 25 13:50:17 2017 -0400
+++ b/mercurial/chgserver.py	Sat Mar 25 21:12:00 2017 -0400
@@ -193,6 +193,7 @@
         def _runpager(self, cmd):
             self._csystem(cmd, util.shellenviron(), type='pager',
                           cmdtable={'attachio': attachio})
+            return True
 
     return chgui(srcui)
 

--- a/mercurial/ui.py	Sat Mar 25 13:50:17 2017 -0400
+++ b/mercurial/ui.py	Sat Mar 25 21:12:00 2017 -0400
@@ -856,15 +856,22 @@
 
         self.debug('starting pager for command %r\n' % command)
         self.flush()
-        self.pageractive = True
-        # Preserve the formatted-ness of the UI. This is important
-        # because we mess with stdout, which might confuse
-        # auto-detection of things being formatted.
-        self.setconfig('ui', 'formatted', self.formatted(), 'pager')
-        self.setconfig('ui', 'interactive', False, 'pager')
+
+        wasformatted = self.formatted()
         if util.safehasattr(signal, "SIGPIPE"):
             signal.signal(signal.SIGPIPE, _catchterm)
-        self._runpager(pagercmd)
+        if self._runpager(pagercmd):
+            self.pageractive = True
+            # Preserve the formatted-ness of the UI. This is important
+            # because we mess with stdout, which might confuse
+            # auto-detection of things being formatted.
+            self.setconfig('ui', 'formatted', wasformatted, 'pager')
+            self.setconfig('ui', 'interactive', False, 'pager')
+        else:
+            # If the pager can't be spawned in dispatch when --pager=on is
+            # given, don't try again when the command runs, to avoid a duplicate
+            # warning about a missing pager command.
+            self.disablepager()
 
     def _runpager(self, command):
         """Actually start the pager and set up file descriptors.
@@ -874,7 +881,7 @@
         """
         if command == 'cat':
             # Save ourselves some work.
-            return
+            return False
         # If the command doesn't contain any of these characters, we
         # assume it's a binary and exec it directly. This means for
         # simple pager command configurations, we can degrade
@@ -891,7 +898,7 @@
             if not fullcmd:
                 self.warn(_("missing pager command '%s', skipping pager\n")
                           % command)
-                return
+                return False
 
             command = fullcmd
 
@@ -904,7 +911,7 @@
             if e.errno == errno.ENOENT and not shell:
                 self.warn(_("missing pager command '%s', skipping pager\n")
                           % command)
-                return
+                return False
             raise
 
         # back up original file descriptors
@@ -925,6 +932,8 @@
             pager.stdin.close()
             pager.wait()
 
+        return True
+
     def interface(self, feature):
         """what interface to use for interactive console features?